Investigating magnetically-induced distortions of neutron stars through gamma-ray burst X-ray plateaus

Abstract

Magnetic field may distort neutron stars (NSs), but the effect has not been robustly tested through gravitational-wave observation yet due to the absence of a fast rotating Galactic magnetar. The central objects of Gamma-ray bursts (GRBs) could be millisecond magnetars. Under the magnetar scenario on the X-ray plateaus of GRB afterglows, the spindown evolution modulated by the gravitational-wave radiation may be inferred from some special samples, so that the magnetically-induced distorting can be further estimated. According to two samples, GRB 060807 and GRB 070521, we found that the correlation between the effective ellipticity, ε B,eff , and effective dipole magnetic field strength on a neutron star (NS) surface, B eff , is ε B,eff ∼ 10 −4 ( B eff 10 14 G ) 2 . This result demands that B eff ∼ 0.01B t with B t being the internal toroidal magnetic field strength of NSs. We suggested that the torque generated during few unsymmetrical massive-star collapses may induce differential rotations in proto-NSs to amplify the internal toroidal fields.